Irradiation treatment by means of X-rays, γ-rays, etc. has been common for the purpose of sterilization of medical equipments. In recent years, such treatment is applied also to blood for transfusion to prevent a graft-versus-host disease (GVHD) by blood transfusion.
To examine whether or not a necessary amount of radiation has been irradiated to the object, it is common to adopt a method wherein an indicator containing a substance which undergoes a color change irreversibly when irradiated with a radiation, is permitted to be present in the object to be irradiated, and after the irradiation, it is taken out, and the dose is ascertained by the color change.
For example, as an indicator for radiation sterilization of medical equipments, one employing an oxidation-reduction dye such as a leuco dye and a polyvinyl chloride, has been practically used. This indicator undergoes a color change when irradiated with a radiation of at least 5000 Gy.
However, the irradiation dose to blood for transfusion is usually at a level of from 15 to 50 Gy, and therefore, the presence or absence of such irradiation can not be detected by this indicator.
JP-A-2-201440 discloses, as an indicator which undergoes a color change when irradiated with a radiation of from 15 to 50 Gy, a radiation color-changeable composition having a metal such as calcium doped on an alkali metal halide such as potassium chloride. However, it is generally known that a halide of an alkali metal is weak to moisture and has been disadvantageous for use in a medical field where it is often in contact with moisture. Further, it has had a drawback that the color-changed portion tends to undergo color fading due to an environmental light such as a room light.
JP-A-2000-65934 discloses a dose meter comprising an organic compound showing an electron accepting property by a radiation and a color-forming electron releasing organic compound. However, the disclosed electron releasing organic compound is generally known to be susceptible to the influence of moisture or impurities in air, and there has been a drawback that the dose meter employing it is poor in the storage stability. Further, such an indicator can not be decolored and in many cases can not be re-used.
It has been proposed to use, for a dose meter, a photochromic compound which shows sensitivity to a radiation and which is easy to handle.
For example, JP-A-2-216493 discloses a radiation-sensitive display sheet made of a laminate having a layer containing a scintillator which emits fluorescence and a layer containing a diarylethene compound which undergoes a color change in response to the fluorescence emitted by this scintillator. However, a maleic anhydride type diarylethene compound such as 2,4,5-trimethylthienylmaleic anhydride used in this reference is inadequate in the sensitivity to a radiation. Further, the maleic anhydride moiety undergoes hydrolysis by moisture, and there has been a drawback that the storage stability is poor.
JP-A-11-258348 discloses a dose meter employing a heat irreversible diarylethene compound. By using the thermally stable and reversible diarylethene compound disclosed in this publication, even a radiation having a relatively low dose can accurately be measured. However, it has been necessary to increase the thickness substantially, in order to measure the radiation at a low dose at a level of from 15 to 50 Gy which is a common dose to blood for transfusion.
It is an object of the present invention to provide an indicator which can be used without being influenced by the environment and which has a good storage stability and is capable of detecting a radiation even at a low dose.